Best thing to do is to, once you become king, or even before you come king try and save as much gold as possible and invest it into local shops/stalls and houses. Smash up the rent to the highest, people will hate you for it but it's going to help them in the long run. Make sure you fill up your treasury to 6.5 million gold (Max capacity) and then make sure you save another few million in your personal treasury to fulfill any promises that you want to keep.
Tip: There is a demon door located in sunset house I think it's called. You have to travel through Mouringwood to get there, but the demon door will open to the king, and inside is one million gold.
Hope this helped.